Because the government made it that way. When the Iranian monarchy was overthrown in 1979, the leaders of the new Islamic republic drew attention to a tenet of the Quran that encourages early marriage and large families. Population growth became part of the national agenda, with incentives to reward families for each additional child. Everything from TVs to cars to food was distributed on a per capita basis through a rationing system, making it advantageous to have many children. These incentives remained in place through the 1980-88 Iran-Iraq War, when population growth was viewed as a strategic advantage: more children, more future soldiers.

The government-spurred baby boom came to an abrupt end toward the end of the war. In 1988, the Ministry of Health and Medical Education held a three-day seminar in Mashad on development, which determined that the rapidly increasing population would soon exceed Iran's infrastructure and natural resources. New family-planning initiatives were supported by members of both the religious and legal communities, enabling the government to make birth control a major policy focus moving forward.

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Although the Ayatollah had issued a fatwa legitimizing contraceptives in 1979, it was only 10 years later—after the pivotal Mashad conference—that they became free and widely available to women in both rural and urban areas. In addition to providing contraception, government-sponsored planning programs encouraged Iranians to space out births and limit family size to no more than three children. Families that ignored these policies were subject to restrictions on paid maternity leave and other government benefits.

Birth rates have decreased radically since the early '90s, so as the Iranian boomers grow older, they will continue to be the largest population group. The percentage of married Iranian women who use some form of contraception rose from 37 percent in 1976 to 74 percent by the year 2000. And there were approximately three times as many children born in 1988 as in 2008.

While Iran's population is young in comparison with the United States', it's fairly old in comparison with other Islamic Middle Eastern countries: Approximately 70 percent of Iraqi and Afghan citizens are under 30, for example. Although other Middle Eastern countries have also tried family planning, they have not achieved the same results as Iran. One possible explanation for this discrepancy is that Iran has a far higher literacy rate than neighboring countries, and literate women are better able to inform themselves about birth control options.